CHI3L1: An Emerging Player in Neuroinflammation and Neurodegeneration.

Neuroinflammation is now being identified as the major factor in the development of various neurological disorders. It is a vital process in neurons and the brain that maintains homeostasis under normal and healthy conditions. However, in hyperactivated states, neuroinflammation can also go awry when microglia and astrocytes enter a toxic, reactive state that can release chemicals that damage neurons. When innate immune cells encounter pathogens, infection, cell debris, or misfolded proteins, they release certain chemokines and cytokines to eliminate the intruding particles and protect the brain. However, persistent inflammatory reactions are harmful and can lead to neurodegeneration by continuously releasing toxic chemicals and proteins. Chitinase-3-like protein 1 (CHI3L1), a secretory protein, is emerging as a key inflammatory molecule that is strongly upregulated during neuroinflammation and has been implicated in the pathogenesis of many diseases. The brain's activated astrocytes are the main source of CHI3L1 and are a dependable biomarker for inflammatory pathologies affecting the central nervous system (CNS), including neurodegeneration and autoimmune diseases. The protein has been implicated in many neurological disorders, including Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis, and others, mediating neuroinflammation and neurodegeneration. CHI3L1 has contrasting functions in the CNS and other tissues. While the protein promotes cell proliferation and migration in various non-neuronal cancers, at the same time, it simultaneously promotes neurodegeneration and apoptosis in the CNS. This paper reviews the current developments in our knowledge of the pathogenic role of the CHI3L1 protein in various neurological disorders.